1. Field of the Invention
The present invention relates to a probe microscope to be used with vibration at a frequency close to a resonance frequency of a cantilever, and more particularly, to a method of measuring vibration characteristics of a cantilever.
2. Description of the Related Art
In recent years, owing to progress in nanotechnology using a semiconductor process, cantilevers having leaf spring characteristics are used in various devices and sensors to conduct various measurements for shape observation, mass, viscoelasticity, magnetic force, and the like. A scanning probe microscope (SPM) includes a cantilever supported in a cantilever state. A surface of a sample is scanned with a probe provided at a tip end of the cantilever. A tunnel current, an interatomic force, a magnetic force, or viscoelasticity, which acts between the probe and the sample, is measured as the amount of bending (displacement) of the cantilever. Therefore, surface shapes or physical properties of the sample may be measured for imaging, and hence the cantilever is used in various fields.
There are proposed many measuring methods using a dynamic force mode (DFM) for detecting a weak force or an interaction with high sensitivity, by vibrating the cantilever of the scanning probe microscope at a frequency close to the resonance frequency of the cantilever, so as to measure an amplitude, a phase, frequency variation, or the like of the vibration.
When measuring frequency-amplitude characteristics (Q curve), an excitation frequency of the cantilever to be used is swept in a range including the resonance frequency thereof in a time related to a response property thereof so as to measure the frequency-amplitude characteristics (Q curve) for detecting the resonance frequency. When the sweep is performed in a long period of time, a frequency at the largest amplitude is liable to match with the resonance frequency. Therefore, the frequency at the largest amplitude can be detected as the resonance frequency (for example, see Japanese Patent Application Laid-open No. Hei 07-174767). An example of conventional and general measurement of frequency-amplitude characteristics is described below.
FIG. 7 is a diagram illustrating a procedure of the conventional and general measurement of frequency-amplitude characteristics in the air. In the air, through use of a common cantilever manufactured in accordance with design values of a resonance frequency of 30 kHz and a Q factor of approximately 100, (i) measurement is performed first in a frequency range of from 20 to 40 kHz with a sweep time of 2 seconds, then (ii) measurement is performed in a frequency range of 4 kHz with the largest amplitude as the center, with a sweep time of 2 seconds, further (iii) measurement is performed at the end in a frequency range of 1 kHz with a sweep time of 2 seconds to enhance accuracy, and (iv) a frequency at the largest amplitude is detected, so as to measure the resonance frequency (see FIG. 9). In this case, the total sweep time of 6 seconds is necessary.
FIG. 8 is a diagram illustrating a procedure of the conventional and general measurement of frequency-amplitude characteristics in a vacuum. Through use of a common cantilever having a design value of a resonance frequency of 300 kHz and an expected value of Q factor of approximately 30,000, which is manufactured in a vacuum, (i) the frequency-amplitude characteristics is measured first in a frequency range of from 200 to 400 kHz with a sweep time of 60 seconds, then (ii) measurement is performed in a frequency range of 40 kHz with the last detected largest amplitude as the center, with a sweep time of 60 seconds, (iii) measurement is performed in a frequency range of 4 kHz with the last detected largest amplitude as the center, with a sweep time of 60 seconds, (iv) measurement is performed in a frequency range of 400 Hz with the last detected largest amplitude as the center, with a sweep time of 60 seconds, (v) measurement is performed in a frequency range of 40 Hz with the last detected largest amplitude as the center, with a sweep time of 60 seconds, and (vi) a frequency at the largest amplitude is detected, so as to measure the resonance frequency. In this case, the total sweep time of 300 seconds is necessary. Because the Q factor is large in the case of the measurement is performed in a vacuum, the sweep time that is several ten to several hundred times the sweep time in the air is necessary.
When the Q factor of resonating is big, the sweep time is lengthened, and it must be in steady state vibration to detect a precise resonance frequency. When the resonance frequency and the Q factor of the cantilever are unknown, the sweep time and the frequency range cannot be appropriately estimated. Therefore, it is necessary to measure the frequency-amplitude characteristics under the condition of sweeping slowly in a wide frequency range. Even when the design values of a resonance frequency and a Q factor of the cantilever in the air are known, a resonance frequency and a Q factor for use in a solution have largely different values, which are thus difficult to estimate. In addition, the resonance frequency in a vacuum has a value similar to that in the air, but the Q factor in a vacuum is apt to be several ten to several hundred times that in the air, which requires a long time for measurement. In addition, it is difficult to estimate the value.
In addition, because a response delay occurs in a short time sweep, an error occurs in detecting the resonance frequency. When the Q factor is large (in a vacuum or in a light gas), the error is apt to increase.
It is known that when an SPM is used in a vacuum, viscosity resistance due to the air is not generated, and the Q factor becomes approximately several ten to several hundred times that in the air. Therefore, measurement of frequency-amplitude characteristics in a vacuum requires a much longer sweep time that is approximately several ten to several hundred times that in the air.
Therefore, when the conventional measurement of frequency-amplitude characteristics is performed on a cantilever having unknown vibration characteristics, it is necessary to judge whether to perform the measurement taking a long period of time for safety or to perform low accuracy measurement in a short period of time. Thus, it is difficult to perform high accuracy measurement in a short period of time.
In addition, as another problem, even when the measurement is performed in a long period of time, a plurality of peaks may often occur in a vicinity of a primary resonance frequency of the cantilever, and hence a true resonance frequency may not be selected.
In addition, when the excitation frequency is swept to activate a vibrator so as to vibrate the cantilever, peripheries of the vibrator that are mechanically connected to the vibrator (a cantilever holder, a slope block, and the like) are also vibrated so that a secondary vibration may occur. The secondary vibration may also affect vibration of the cantilever, and may generate an amplitude peak at a frequency other than the resonance frequency of the cantilever. Then, a detection error of the resonance frequency of the cantilever may be caused.